The human interleukin-2 receptor is being studied to understand critical components of the T cell immune response in normal and neoplastic cells. Following T-cell activation, IL-2 and IL-2 receptors are induced; the magnitude and duration of the T-cell immune response is controlled by the amount of IL-2 produced, the levels of receptors expressed, and the time course of these events. Three chains of the IL-2 receptor exist, IL- 2Ra, IL-2Rb, and gc, with IL-2Ra and IL-2Rb being significantly regulated at the level of transcription. The laboratory has focused primarily on the types of signals induced by IL-2, particularly the activation of STAT proteins, and the mechanism of regulating IL-2Ra gene expression in response to mitogen and IL-2. Considerable progress has been made in analyzing the STAT proteins (signal transducers and activators of transcription) induced by IL-2. IL-2 can activate both Stat5a and Stat5b (two closely related proteins with <90% amino acid identity) in fresh peripheral blood lymphocytes (PBL) and additionally activates Stat3 in PBL preactivated with phytohemagglutinin. Consistent with the presence of an IL-2 response element in the 5? regulatory region of the IL-2Ra gene, it was demonstrated that Stat5a knockout mice are defect in IL-2-induced IL-2Ra expression and that this is associated with defective superantigen-induced T-cell expansion in vivo and defective IL-2-induced proliferation in vitro. This defective proliferation could be overcome by doses of IL-2 high enough to titrate intermediate affinity IL-2 receptors. Interestingly, Stat5b knockout mice also exhibit defective IL-2-induced IL-2Ra expression, but in this case high dose IL-2 cannot normalize proliferation. Stat5b knockout mice also exhibit a substantial defect in the proliferative and cytolytic activities of natural killer cells. These studies substantially extend our understanding of the molecular basis for IL-2 upregulation of the IL-2Ra gene and partially clarify the roles of Stat5a and Stat5b. Purified Stat5a and Stat5b proteins were purified using a baculovirus expression system so that the requirements for binding and activation of these proteins can be studied with purified reagents, and a binding site-selection analysis is in progress. The yeast two hybrid method was used to identify factors that can interact with Stat5, and two interacting proteins have been identified that appear to clarify the basis for Stat5 function. Together, these studies substantially enhance our understanding of the basis for IL-2R expression as well as IL-2-dependent gene regulation.